In recent years, blue lasers permitting ultrahigh density recording have been rapidly developed and WORM (write-once read-many) optical recording media compatible therewith are under development. Among others, there are strong demands for development of a dye coating type WORM medium enabling efficient production at relatively low cost. In a conventional dye coating type WORM optical recording medium, a laser beam is applied to a recording layer of an organic compound containing a dye as a main component, to mainly cause an optical (refractive index or absorptance) change based on decomposition or alteration of the organic compound, thereby forming a recording pit. The recording pit part does not involve only the optical change, but also it normally involves deformation based on a change in volume of the recording layer, formation of a mixed region of a substrate and the dye due to generation of heat, deformation of the substrate (mainly, a rise due to expansion of the substrate), and so on (cf. Patent Document 1, Patent Document 2, Patent Document 3, and Patent Document 4).
Factors important for formation of a good recording pit include optical behavior of the organic compound used in the recording layer, to a wavelength of a laser used for recording or reproduction, and thermal behavior thereof such as decomposition or sublimation, and generation of heat occurring therewith. Therefore, the organic compound used in the recording layer is selected from materials with appropriate optical properties and decomposition behavior.
In the first place, the conventional WORM media, particularly, CD-R and DVD-R are designed to achieve a reflectance of about 60% or more and, similarly, a high modulation index over about 60%, in order to maintain reproduction compatibility with read-only recording media (ROM media) in which depressed pits preliminarily formed on a substrate are covered by a reflecting layer of Al, Ag, Au, or the like. First, the optical properties of the recording layer are defined for achieving a high reflectance in an unrecorded state. Normally, required values are the refractive index n of at least about 2 and the extinction coefficient of approximately from 0.01 to 0.3 in the unrecorded state (cf. Patent Document 5 and Patent Document 6).
With a recording layer containing a dye as a main component, it is difficult to achieve a high modulation index of at least 60% by only change in the optical properties due to recording. Namely, a dye as an organic substance can exhibit limited change amounts of the refractive index n and absorptance k, and there is thus a limit to change in the reflectance in a planar state.
Therefore, a method used is to apparently increase reflectance change (reflectance decrease) in the recording pit part, using interference effect between reflections from two parts based on a phase-difference between reflected light from the recording pit part and reflected light from an unrecorded part. In other words, the method is based on a principle similar to that of phase-difference pits as in the ROM media, and in the case of the organic recording layer with change in refractive index smaller than those of inorganic substances, it is reported that it is rather advantageous to mainly use the reflectance change based on the phase-difference (cf. Patent Document 7). Furthermore, there is a study with comprehensive consideration to the above-mentioned recording principle (cf. Non-patent Document 1).
In the description hereinafter, a portion recorded as described above (which is sometimes called a recording mark part) will be referred to as a recording pit, a recording pit part, or a recording pit portion, irrespective of its physical shape.
FIG. 1 is a drawing to illustrate a WORM medium (optical recording medium 10) with a recording layer containing a dye as a main component in a conventional configuration. As shown in FIG. 1, the optical recording medium 10 has the configuration in which at least a recording layer 12, a reflecting layer 13, and a protecting coat layer 14 are formed in the order mentioned, on a substrate 11 with a groove therein, and an objective lens 18 is used to introduce a recording/reproducing light beam 17 into and through the substrate 11 to irradiate the recording layer 12. The thickness of the substrate 11 is normally 1.2 mm (CD) or 0.6 mm (DVD). A recording pit is formed in a portion of a substrate groove part 16 called an ordinary groove, on the near side to a surface 19 which the recording/reproducing light beam 17 enters, but is not formed in a substrate land part 15 on the far side.
It is also reported as to the phase-difference change in the aforementioned background art document that factors contributing to the phase-difference change are maximization of refractive index change between refractive indices before and after recording in the recording layer 12 containing the dye, and shape change of the recording pit part, i.e., effect of local change in the shape of the groove (equivalent change in the depth of the groove due to expansion or depression of the substrate 11) or change in film thickness (transmissive change in film thickness due to expansion or constriction of the recording layer 12), in the recording pit part formed in the groove.
In the recording principle as described above, the wavelength of recording/reproducing light is normally selected as located in a longer-wavelength-side tail of a large absorption band, in order to enhance the reflectance in the unrecorded state and to induce a large refractive index change by decomposition of the organic compound with laser irradiation (which achieves a large modulation index). This is because the longer-wavelength-side tail of the large absorption band is a wavelength region realizing an appropriate extinction coefficient and a large refractive index.
However, there has been no available material with values of the optical properties comparable to those of the conventional materials for the wavelengths of the blue lasers. There are very few organic compounds with the optical constants comparable to those required of the recording layer in the conventional WORM optical recording media, particularly, near 405 nm, which is the center of oscillation wavelengths of the blue semiconductor lasers now in practical use, and such a material is still in a stage of search. Furthermore, in the case of the WORM optical recording media having the conventional dye recording layer, a main absorption band of the dye exists near the wavelength of recording/reproducing light, and therefore the optical constants thereof come to have a large wavelength dependence (i.e., the optical constants largely vary depending upon wavelengths), which raises a problem of significant change in recording characteristics such as recording sensitivity, modulation index, jitter, and an error rate, in reflectance, and so on against variation in the wavelength of recording/reproducing light due to individual differences of lasers, change in ambient temperature, and so on.
For example, there is a report on an idea of recording using a dye recording layer showing absorption near 405 nm, but the dye used therein is required to have the same optical characteristics and functions as those of conventional dyes; therefore, it is entirely dependent on search and discovery of a high-performance dye (cf. Patent Document 8 and Patent Document 9). Furthermore, it is reported as to the WORM optical recording medium 10 using the recording layer 12 containing the conventional dye as a main component, as shown in FIG. 1, that it is also necessary to appropriately control a groove shape and a distribution of thicknesses of the recording layer 12 in the substrate groove part 16 and in the substrate land part 15 (cf. Patent Document 10, Patent Document 11, and Patent Document 12).
Namely, in terms of securing the high reflectance as described above, it is possible to use only a dye with a relatively small extinction coefficient (approximately from 0.01 to 0.3) for the wavelength of recording/reproducing light. For this reason, it is impossible to decrease the film thickness of the recording layer 12, in order to obtain optical absorption necessary for recording in the recording layer 12 and to achieve a large change in the phase-difference between phases before and after recording. As a consequence, the film thickness of the recording layer 12 is normally determined to be a thickness approximately equal to λ/(2 ns) (where ns represents the refractive index of the substrate 11), and it is desirable to use the substrate 11 with a deep groove, in order to embed the dye of the recording layer 12 in the groove and to reduce crosstalk.
Since the recording layer 12 containing the dye is normally formed by a spin coat method (coating method), it is actually convenient to embed the dye in the deep groove and to increase the thickness of the recording layer 12 in the groove part. On the other hand, the coating method results in making a difference between film thicknesses of the recording layer in the substrate groove part 16 and in the substrate land part 15, and this difference between film thicknesses of the recording layer is advantageous in that a stable tracking servo signal is obtained even with use of the deep groove.
Namely, it is infeasible to maintain a good signal characteristic in the recording pit part and a good tracking signal characteristic together unless the groove shape defined by the surface of the substrate 11 and the groove shape defined by the interface between the recording layer 12 and the reflecting layer 13 in FIG. 1 are kept both at appropriate values.
The depth of the groove is normally determined to be preferably close to λ/(2ns) (where λ represents the wavelength of the recording/reproducing light beam 17 and ns the refractive index of the substrate 11) and is determined in the range of about 200 nm for CD-R and about 150 nm for DVD-R. It is very difficult to form the substrate 11 with such a deep groove and this is a cause to degrade the quality of the optical recording medium 10.
Particularly, in the case of the optical recording medium used with the blue laser beam, when λ=405 nm, it is necessary to use a deep groove of approximately 100 nm and it is often the case that the track pitch is set in a range of from 0.2 μm to 0.4 μm, in order to achieve a high density. It is much more difficult to form such a deep groove at the narrow track pitch and it is practically impossible to realize mass production with conventional polycarbonate resin. Namely, in the case of the medium used with the blue laser beam, mass production is quite likely to become impossible in the conventional configuration.
Furthermore, the examples described in the aforementioned background art documents are mostly examples using the conventional configuration (substrate incidence configuration) represented by the optical recording medium 10 shown in FIG. 1. In order to realize high-density recording with the blue laser, however, attention is being drawn to a configuration so called film-surface-incidence and there is a report on a configuration using an inorganic material recording layer such as a phase-change type recording layer (cf. Non-patent Document 3).
In the configuration called the film-surface-incidence, opposite to the conventional configuration, at least a reflecting layer, a recording layer, and a cover layer are formed in the order mentioned, on a substrate with a groove therein, and a focused laser beam for recording/reproduction is introduced into and through the cover layer to irradiate the recording layer.
The thickness of the cover layer is normally approximately 100 μm in a so-called Blu-Ray Disc (cf. Non-patent Document 9). The reason why the recording/reproducing light is introduced from such a thin cover layer side is that the objective lens used for focusing of the beam has a numerical aperture (NA which is normally from 0.7 to 0.9, and is 0.85 for the Blu-Ray Disc) higher than those before. With use of the objective lens having the high NA (numerical aperture), the required thickness of the cover layer is as thin as approximately 100 μm, in order to keep down influence of aberration due to the thickness of the cover layer. There are many reports of examples about such blue wavelength recording and film-surface-incidence configuration (cf. Non-patent Document 4 and Patent Documents 13-24). There are also many reports on related technologies (cf. Non-patent Documents 5-8 and Patent Documents 25-43).
Non-patent Document 1: Proceedings of International Symposium on Optical Memory, U.S.A., Vol. 4, 1991, pp. 99-108
Non-patent Document 2: Japanese Journal of Applied Physics, Japan, Vol. 42, 2003, pp. 834-840
Non-patent Document 3: Proceedings of SPIE, U.S.A., Vol. 4342, 2002, pp. 168-177
Non-patent Document 4: Japanese Journal of Applied Physics, Japan, Vol. 42, 2003, pp. 1056-1058
Non-patent Document 5: Compact Disc Technology, co-authored by Heitaro Nakajima and Hiroshi Ogawa, revised third edition, Ohmaha, 1996, p. 168
Non-patent Document 6: Japanese Journal of Applied Physics, Japan, Vol. 42, 2003, pp. 914-918
Non-patent Document 7: Japanese Journal of Applied Physics, Japan, Vol. 39, 2000, pp. 775-778
Non-patent Document 8; Japanese Journal of Applied Physics, Japan, Vol. 42, 2003, pp. 912-914
Non-patent Document 9: Optical Disc Kaitai Shinsho, edited by NIKKEI ELECTRONICS, Nikkei Business Publications, Inc., 2003, third chapter
Non-patent Document 10: Spectral Ellipsometry, authored by Hiroyuki Fujiwara, Maruzen Company, 2003, fifth chapter
Non-patent Document 11: Adhesive and Adhesion Technology Introductory Book authored by Alphonsus V. Pocius, and translated by Hiroshi Mizumachi, Hirokuni Ono, THE NIKKAN KOGYO SHINBUN, LTD., 1999
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